The present invention relates to the preparation of 2,2,2-trifluoroethanol and 1,1,1,3,3,3-hexafluoroisopropanol, and more particularly, it relates to the preparation of such fluorinated alcohols by hydrogenolysis of hydrates or hemiacetals of the corresponding polyfluorinated carbonyl compounds.
A major commercial source of fluorinated primary alcohols such as 2,2,2-trifluoroethanol is based on the reduction of the corresponding acid (trifluoroacetic acid in this case) or of a derivative (the ester, acid chloride, anhydride, or amide) with hydrogen in the presence of a catalyst generally chosen from the precious metals group, such as rhodium, ruthenium, platinum, or palladium. This trifluoroethanol is used in a wide variety of applications including energy recovery, in absorption type heat pumps; pharmaceutical products, such as anaesthetics; and as solvents.
The hydrogenation of trifluoroacetic anhydride is shown in U.S. Pat. No. 4,255,594and that of trifluoroacetic acid in U.S. Pat. No. 4,273,947; hydrogenation of esters of trifluoroacetic acid shown in European Pat. No. 36,939; hydrogenation of trifluoroacetamide shown by M. Gilman, J.A.C.S., 70, 1281-2 (1948); and hydrogenolysis of trifluoroacetyl chloride as shown in U.S. Pat. No 3,970,710 are some of the usual methods employed to prepare fluorinated primary alcohols. In addition to the disadvantage of catalyst activity decreasing with the passage of time, these processes share the economic disadvantage of resorting to an oxidation of the usually chlorine-containing starting materials to obtain the acid or one of its derivatives, followed by a reduction of this acid to the alcohol. This additional step very severely impairs the profitability of these processes.
Another group of processes calls for hydrogenating trifluoroacetaldehyde (called "fluoral" hereinafter) or one of its derivatives. The yield obtained by liquid phase hydrogenation (80.degree. C. at 95 bars) of fluoral hydrate over Raney nickel as shown in French Pat. No. 1,399,290 is mediocre. The catalyst life and the purity required in the starting material are not stated. There is a risk of a side reaction, known as the haloform reaction: EQU CF.sub.3 CHO+NaOH.fwdarw.CHF.sub.3 +HCOONa (1)
with the sodium hydroxide which is always absorbed on the catalyst.
U.S. Pat. No. 2,982,789 describes gas-phase hydrogenation of fluoral hydrochloride, CF.sub.3 CH(Cl)OH, obtained from a first hydrogenolysis step (the Rosenmund reaction) of trifluoroacetyl chloride on a palladium catalyst. The fluoral hydrogenation catalyst, which consists of copper chromite deposited on calcium fluoride, and whose behaviour with the passage of time is not mentioned, operates at about 250.degree. C. and permits the intermediate fluoral to be converted only incompletely (approximately 60-65%). Additionally, recycling of the unconverted fluoral hydrochloride is a very risky operation because of its thermal instability, the decomposition: ##STR2## being promoted even at 30.degree. C. by a temperature rise or a pressure reduction. Finally, U.S. Pat. No. 3,468,964 shows gas phase hydrogenation of fluoral in the presence of a catalyst containing palladium deposited on alumina, at low temperature (peak temperature: 140.degree. C.). The moderate yield of trifluoroethanol (86%) and the need to regenerate the catalyst very frequently at 200.degree. C. in pure oxygen, together with the extreme difficulty of handling fluoral in the pure state, due to polymerization, make the process unattractive.
French Pat. No. 2,027,172 relates to a process for gas phase hydrogenation of perhaloketones over a palladium-based catalyst deposited on activated charcoal and particularly describes catalytic hydrogenation of hexafluoroacetone (CF.sub.3 COCF.sub.3) to yield 1,1,1,3,3,3-hexafluoroisopropyl alcohol. The yield obtained (75%) and the difficulty of handling the starting material (b.p.: -27.4.degree. C./760 torr) make the process uneconomical. French Pat. No. 2,479,803 carries out a gas phase catalytic hydrogenation of hexafluoroacetone to 1,1,1,3,3,3-hexafluoroisopropanol over a nickel-based catalyst. The catalyst life is not rated for a continuous operation time longer than 20 hours and, above all, this process requires the use of a pure starting material, which is difficult to obtain in the case of hexafluoroacetone. French Pat. No. 1,361,260 carries out the gas phase catalytic hydrogenation of hexafluoroacetone over a catalyst based on copper chromite. The yields of 1,1,1,3,3,3-hexafluoroisopropanol are mediocre (approximately 40%) for a partial conversion (83%) of the ketone used. This entails difficult and costly recycling.
French Pat. No. 2,133,126 describes a liquid phase process for the hydrogenation of perfluoroacetone to hexafluoroisopropanol in the presence of a catalyst containing palladium activated with an inorganic base of the alkali metal type. The catalyst so activated makes it possible to obtain partial conversion (80 to 86%) of the starting ketone, but the hydrogenation times are long (approximately seven hours) and the hexafluoroacetone employed must be purified, which calls for a preliminary distillation at elevated pressure and complicates the process. The catalyst life is not specified.
French Pat. No. 2,493,831 shows a gas phase process for hydrogenating hexafluoroacetone hydrate in the presence of a catalyst based on nickel or palladium, excluding ruthenium or platinum. This process has a major disadvantage in that it is absolutely necessary to use a ketone hydrate which is particularly pure and refined by means of a complex method which is the subject of U. K. Pat. No. 2,086,891. This imperative requirement of refining the starting material to a high degree of purity makes the process for producing 1,1,1,3,3,3-hexafluoroisopropanol considerably more complicated.
Finally, Japanese published application 83/88,330 describes a process for the preparation of 1,1,1,3,3,3-hexafluoroisopropanol from hexafluoroacetone in the gaseous, or optionally in liquid, phase in the presence of a catalyst based on rhodium deposited on activated charcoal. Its principal disadvantage lies in the use of a very costly catalyst. In addition, the indication of a very slight drop in yield between the third and the fourth hour of operation (Example 2) from 100% to 98% leads to reservations concerning the actual behaviour of the catalyst with the passage of time. In fact, if this drop continues at the same rate, it means that the residence time must be doubled after every 50 hours of operation; this is not economically feasible.